Process for producing a high-quality insulation for electric conductors or conductor bundles of rotating electrical machines by means of spray sintering

ABSTRACT

The present invention discloses a process for producing a high-quality insulation for conductors or conductor bundles of electrical machines by means of spray-sintering. Unlike in the prior art, it is possible to apply internal corona-discharge protection, insulation and external corona-discharge protection to conductors or conductor bundles in which, on account of a very low level of defects in the insulation and its inherent resistance to partial discharges, it is possible to dispense with the use of winding processes using glass/mica tapes. This eliminates the need to use complex special equipment and allows the throughput times to be shortened considerably.

TECHNICAL FIELD

[0001] The invention relates to the field of the insulation of rotatingelectrical machines. In particular, the invention relates to a processfor producing a high-quality insulation for conductors or conductorbundles as are used in rotating machines, for example in the form ofstator coils, transposed bars and excitation conductors.

PRIOR ART

[0002] Various processes are customarily used in the field of theinsulation of conductors or conductor bundles of rotating electricalmachines.

[0003] In one process, tapes comprising a glass-fiber support and micapaper are wound helically in layers onto a stator conductor until adesired insulation thickness is reached. Subsequent impregnation inepoxy resin displaces residual air from the insulating winding formed inthis way, and the layers of tape are adhesively bonded. Curing in asuitable mold imparts the final shape to the insulation. For productionreasons, in this process the mica platelets are oriented in thedirection of the tape, which in the finished insulation results in themica platelets being oriented parallel to the conductor surface. In theresin rich technique, epoxy resin in the B state is admixed with thetape and is consolidated by hot pressing of the bar.

[0004] According to a further process, which is known from EP 0 660 336A2, tapes consisting of thermoplastic filled with mica are wound aroundthe stator conductor. Consolidation and shaping in this case take placeby means of hot pressing of the stator conductor around which the tapehas been wound, during which process air is displaced, the thermoplasticis melted and the layers of the winding are adhesively bonded. In thisprocess too, the mica platelets are oriented parallel to the conductorsurface. However, the air is not completely expelled in any of theprocesses. Air-filled gaps and holes remain, in which, in the event of avoltage load, partial discharges in the nC range and above occur.

[0005] Finally, the stator conductor can also be insulated by extrusionof thermoplastics without fillers, i.e. also without mica, as describedin U.S. Pat. No. 5,650,031.

[0006] Nowadays, however, the conductors of rotating electrical machineswhich are to be insulated are generally structures of a very complexshape, in the form of bars or coils. A straight part of the conductorsis located in the grooves of the stator of the machine. A curved part ofthe conductors, after suitable connection to adjacent bars and coils,forms a winding head which projects out of the stator at both ends. Inthe case of large rotating machines, the length of the straight part mayexceed 6 m. A problem hitherto has been that insulation and conductorusually have different coefficients of thermal expansion α which, overthe course of time, on account of thermal stresses, may lead to defectsin the insulation as a result of cavities which form where theinsulation becomes detached, and that defects, for example inclusions ofair, are formed during the production of the insulation. Partialdischarges may occur at such defects, leading to damage to theinsulation. In this case too, partial discharge activities in the 100 nCrange are quite customary.

[0007] In view of these partial discharge activities, hitherto it hasonly been possible for the machine insulation to operate reliably as aresult of the barrier action of mica platelets oriented perpendicular tothe field direction. This prevents the formation of flashover passagesleading out of the cavities. 2.5 to 2.75 kV/mm is generally regarded asthe upper limit for long-term reliability of the operating fieldstrength. However, a maximum level such as this is exceeded, in somecases considerably, by other insulation systems used in medium- orhigh-voltage insulation.

[0008] For example, the maximum field for long-term operation inpin-type insulators, in which an alumina-filled epoxy resin is used forgas-insulated circuits, is 4 kV/mm, and the maximum field forhigh-voltage cables, in which polyethylene is used, is approx. 12 kV/mm.A common feature of these conventional insulation systems is that thereare no partial discharges under operating load.

[0009] However, since, moreover, the conventional processes andmaterials using mica which are currently in use are substantiallyalready more than thirty years old, at best incremental improvements areto be expected from any further developments to this prior art.Therefore, it appears highly unlikely that it will be possible tofurther develop this prior art to develop a higher-quality insulationwhich can be produced with shorter throughput times and lowermanufacturing costs compared to the prior art, and also in anenvironmentally friendly production process, i.e. without the use ofsolvents, without emissions and without the production of special waste,and which does not include any defects or, if there are defects, thesedefects do not lead to any partial discharges.

SUMMARY OF THE INVENTION

[0010] Therefore, it is an object of the present invention to provide aprocess for producing a high-quality insulation for conductors orconductor bundles in which the insulation has a high quality and can beproduced with short throughput times, low manufacturing costs and in anenvironmentally friendly manner.

[0011] According to the invention, this object is achieved by theprocess for producing a high-quality insulation of conductors orconductor bundles having the features of patent claim 1. Advantageousrefinements of the invention are given in the subclaims.

[0012] This inventive process for producing a high-quality insulationfor conductors or conductor bundles without cavities which may lead topartial discharges under test and operating loads means that theoriented mica platelets are no longer required. This greatly facilitatesboth the choice of production processes and the choice of materials forthe insulation, since for many polymers it is difficult to incorporatemica in concentrations of more than 40% by weight.

BRIEF DESCRIPTION OF THE DRAWING

[0013] The invention is explained in more detail below with reference toa preferred exemplary embodiment which is illustrated in the drawing andin which:

[0014]FIG. 1 shows a structure of the spray-sintering device accordingto the invention, and

[0015]FIG. 2 (FIG. 2-1 and FIG. 2-2) shows a flow diagram whichillustrates the way in which the process according to the invention iscarried out.

WAY(S) OF CARRYING OUT THE INVENTION

[0016] The following text provides an extensive description of a processfor producing a high-quality insulation for conductors or conductorbundles of, for example, rotating electrical machines. First of all, thebasic structure of the insulation will be dealt with, and then theprocess according to the invention will be explained in detail.

[0017] The insulation which is applied using the process according tothe invention comprises three layers. The first layer forms an internalcorona-discharge protection, consisting of conductively orsemiconductively filled polymer. In this case, a polymer which can besuccessfully joined to the polymer material of the insulating layerwhich follows it is used. It is preferable to use the same polymer as inthe insulating layer.

[0018] As is the case in high-voltage cables, the internalcorona-discharge protection has the role of decoupling electrical andmechanical boundary layers. In electrical terms, the internalcorona-discharge protection has the same potential as the metallicconductor below it, i.e. is part of the electrical conductor; inmechanical terms, by contrast, it is part of the insulation. Thisensures that any points where the insulating sleeve and conductor becomedetached are free from partial discharges, since there is no voltagedrop across the detachment.

[0019] The process according to the invention for the production of thishigh-quality insulation for conductors or conductor bundles is intendedto satisfy the following demands:

[0020] 1) The production process is to be substantially independent ofthe particular geometry of the initial bar or coil, i.e. of thetransposed, uninsulated, consolidated bar or coil.

[0021] 2) The insulation is to be of a high quality, i.e. compared tothe prior art is to have an improved thermal stability up to approx.T_(max)=180° C. and is to be able to withstand long-term operation atapprox. 5 kV/mm on the flat sides of the conductors without beingdamaged.

[0022] 3) Furthermore, the process is to allow production of aninsulation of constant thickness with a tolerance Δd/d<10%—even if thetolerances of the initial bar or coil are considerably greater—while itis to be possible to produce layer thicknesses of from 0.3 to 7 mm.

[0023] 4) To shorten the production time, the throughput time per bar orcoil is to be at most 1 to 2 hours.

[0024] In view of these demands which are to be satisfied by the processaccording to the invention, one could consider using conventionalspray-sintering processes as the starting point.

[0025] A conventional spray-sintering process of this type is described,for example, in German patent DE 39 36 431, entitled “Process forapplying a layer of plastic to a metallic conductor”. In thisconventional process, to apply a layer of plastic to a metallicconductor plastic powder is electrostatically charged, is deposited onthe preheated metallic conductor and is then melted. The process is usedto produce subconductor insulations, i.e. insulations with a low voltageload. This conventional spray-sintering process would require asignificantly lower outlay on manufacturing technology than theconventional insulating process which involves winding a wide range oftapes around the conductor. This would make it possible to eliminate theuse of expensive special equipment, such as for example winding robots,vacuum/pressure vessels, devices for the cooled storage of liquid resin.They would be replaced by commercially available coating units andcommercially available robots.

[0026] It would also be possible for this conventional spray-sinteringprocess to be automated to a far greater extent than the conventionalprocess. The throughput times would be only 0.5 to 3 hours, instead ofseveral days. The saving on investment costs would lead to thepossibility of achieving both shorter throughput times and lowerproduction costs.

[0027] However, it has proven to be a problem that the technique ofspray-sintering or electrostatic spraying is currently generally onlyused for the dry painting of a very wide range of components, forexample refrigerators, automobiles, garden furniture, etc., with verylow layer thicknesses of approx. 80 μm. In the field of electricalengineering, this technique has hitherto only been used for theinsulation of busbars in the medium- and high-voltage range. As hasalready been mentioned above, however, the insulation described above isonly used to insulate subconductors in which there are only lowpotential differences.

[0028] In this conventional application in the field of insulation forbusbars, however, unlike the high-quality insulation which is to beproduced by the process according to the invention there is only a weakelectrical load on the insulation, since the ground electrode is absenton the insulation, and therefore the voltage is for the most partreduced via the ambient air. Since, therefore, the electrical load isconsiderably lower, in the prior art the demands with regard toinsulation thickness and freedom from defects are considerably lowerthan for the process according to the invention, and therefore defectsare acceptable since there is no risk of partial discharges. It iscustomary to use a powder which is based on epoxy resin, but in thecurrent composition this powder cannot be used at above T=130°, onaccount of high dielectric losses. Moreover, sample coatings using thispowder have high levels of pores.

[0029] Therefore, compared to the known prior art of spray-sintering,the process according to the invention for the production of ahigh-quality insulation for conductors or conductor bundles requiresnumerous modifications in order to solve problems which have nothitherto been taken into account and to eliminate properties which areadvantageous for this process in its conventional application but causeproblems with a view to achieving the objectives of the invention.

[0030]FIG. 1 diagrammatically depicts a device for carrying out theprocess according to the invention; for the sake of simplicity, someelements are not shown in FIG. 1 but rather are merely described indetail.

[0031] The central appliance for carrying out the process according tothe invention is a spray gun 1. Coating material 2 in powder form, forexample thermally crosslinking epoxy powder, is fed in fluidized form tothis spray gun 1 from a reservoir 6. The fluidizing medium is in thiscase preferably dried air or nitrogen. The spray gun 1 may have a device3 for electrostatically, i.e. negatively, or tribo-electrically, i.e.positively, charging the coating material 2, i.e. the powder particles.

[0032] However, it is not absolutely imperative for the coating material2 to be electrically charged in the process according to the invention.Nevertheless, this charging has the advantage that greater amounts ofmaterial are applied at points and edges of a conductor which is to beinsulated, on account of the local increase in the electric field. Thiscan be influenced by adjusting the high voltage at the device 3. This isoften desirable, since at these points the field strength is also higherin operation, and experience has shown that this is where voltage tendsto break through the insulation.

[0033] In the process according to the invention, the spray gun 1 ismoved past a heated substrate 4 which is to be coated, for example aconductor which is to be coated, at a constant distance, for exampleapprox. 100 to 300 mm, a constant speed, for example approx. 50 to 800mm/s, and with a constant delivery of powder, for example approx. 30 to250 g/min. The spray gun 1 is preferably guided by means of an automaticdisplacement unit, e.g. a robot. The substrate is heated throughout theentire coating process. Since the present example relates to anelectrical conductor, this can easily be achieved by electrical heating.Specifically, the substrate 4 can be heated either by resistance heatingby means of direct current or low frequency, for example at 50 Hz, or byinductive heating by means of medium frequency or high frequency. Forthis purpose, by way of example a current source 5 is provided. Thetemperature of the substrate surface 4 a which is to be coated isselected in such a way that the coating material 2, for example theepoxy powder, melts when it comes into contact with the surface and isthen thermally crosslinked, i.e. cured.

[0034] In addition to the heating during coating which is describedhere, it is also conceivable to use methods in which the object to becoated is preheated (for example in a separate furnace), and is thentransferred to the coating installation. On account of the high heatcapacity of copper, it is now possible for the bar or coil to be coatedwith at least part of the total insulation thickness required. If thesurface temperature drops to such an extent that the powder no longermelts to form a smooth film, but rather adheres in the form of asand-like covering, it is necessary to supply further heat. The questionof whether heating takes place continuously during coating or heatingand coating cycles alternate makes no difference to the quality of thefinished insulation. It is sensible to use the intermediate heating in afurnace if, in the case of objects with a very large Cu cross section,resistive heating or inductive heating are difficult to implement (onaccount of difficulties with introducing the current and contactresistance, or because HF sources in the 15 to 10 kA range are expensiveand have to be shielded at high cost, respectively).

[0035] In an advantageous embodiment, the “furnace” comprises anassembly of IR radiators which is drawn or moved or fitted over theobject mounted on the coating installation.

[0036] In the process according to the invention, the coating takesplace in layers until the desired thickness of the insulation isreached, unlike in the prior art, in which the coating takes place in asingle path. In this process, one layer corresponds to the insulatinglayer which is produced by means of one spray pass. In this case, aftera spray pass, the coating material must be given sufficient time tomelt, run and crosslink sufficiently for it no longer to be able toflow. Therefore, the gel time, which is to be explained in more detailbelow, is of considerable importance.

[0037] In principle, it is generally possible to build up layers with athickness of 1 mm and more in one spray pass. However, tests have shownthat for layer thicknesses of greater than 0.2 mm the number of bubbles,i.e. defects, in the insulation rises considerably, but hitherto thesebubbles have not presented any problems in the application area ofspray-sintering. The reason for the formation of bubbles at layerthicknesses of >0.2 mm is that adsorbed substances and impurities with alow vapor pressure are no longer able to evaporate out freely at highlayer thicknesses. In conventional applications, this formation ofbubbles caused few problems, but it does represent a drawback for theinsulation according to the invention, and this drawback is to beavoided by the process according to the invention. Therefore, in theprocess according to the invention the insulation is applied insuccessive steps, in layer thicknesses of up to 0.2 mm, so that thedesired freedom from defects can be achieved.

[0038] The constant spraying distance, spray-gun speed of movement anddelivery of coating material and powder as explained above results in athickness consistency of better than 0.08 mm for total thicknesses ofapprox. 1 mm. Since the rate can be changed very easily and in a verycontrolled way, the process according to the invention is able tolocally vary the thickness of the insulation. For example, it ispossible to increase the insulation thickness on the narrow sides of thetransposed bar compared to the thicknesses on the wide sides, with theresult that the electrical field strength is reduced without the groovewidth having to be increased or without the dissipation of heat from thebar, which takes place via the wide sides, being impeded.

[0039] Furthermore, with the process according to the invention it isalso possible to reduce the insulating thickness in the bow region oftransposed bars, where the electric field load is generally lower thanin the straight part. An axial variation in the thickness or compositionof conductive or semiconducting layers also leads to possibilities withregard to field dissipation which cannot be achieved or can only beachieved with difficulty using the conventional system.

[0040] If the thickness tolerance referred to above is insufficient orif a complicated layer distribution is desired, it is additionallypossible to adapt the rate of movement of the spray gun, by contactlessmeasurement of the current layer thickness at a given location x, y, zby means of infrared and by suitable feedback to the control system ofthe robot, in such a manner that the final thickness at the location x,y, z corresponds to the desired value.

[0041] In principle, all thermally crosslinkable plastics, known asthermosets, can be used as materials for the insulation. The requirementthat the insulation be thermally suitable for use at up to 180° C. inthe present application is best satisfied by epoxy materials. Thesematerials consist of a mixture of at least one uncrosslinked resin andat least one hardener (as well as a few further additions, such asaccelerators, pigments, etc.) and inorganic fillers. The mixture issolid up to at least 50° C. The melting and curing temperatures and theglass transition temperature T_(g) vary according to the chemicalcomposition of resin and hardener. The temperature profile of themechanical and dielectric strength is closely linked to the glasstransition temperature T_(g). If it is desired for the insulation to beusable for thermal class H, T_(g) should lie in this range, preferablybetween 150° C. and 200° C. Glass transition temperatures ofsignificantly above 200° C. are, on the one hand, difficult to achieveand, on the other hand, lead to a material which is relatively brittlein the region of room temperature.

[0042] The abovementioned, desired freedom from bubbles is dependent notonly on process parameters, such as the application thickness, but alsoon materials properties.

[0043] It is important that the epoxy in the liquid state has asufficiently low viscosity to run and for the gel time to be long enoughfor all the bubble-forming impurities to have evaporated. Thisrequirement for long gel times contradicts the conventional trend inpowder coating where the spray-sintering technique has hitherto beenused, namely that of deliberately establishing short gel times, forexample typically of 15 s, by the addition of accelerators in order toachieve high throughput times during thin-film coating. However, byreducing the level of accelerator, it is possible without difficulty toachieve gel times for commercially available powders of ≧40 s, which issufficiently long for the present application.

[0044] For spray powders, the viscosity is generally not measured andspecified as a separate variable; rather, what is known as the run,resulting from the viscosity and gel time, is specified. Bubble-freelayers are achieved if the run is >30 mm.

[0045] Filling with inorganic fillers is in principle desirable in orderto reduce the price, improve the creep strength, reduce the coefficientof thermal expansion and improve the thermal conductivity of theinsulation. The proportion of filler in the total mixture should amountto 5-50% by weight, based on a closed filler density of up to 4 g/cm³.Examples of conventional fillers are silica flour, wollastonite, talcand chalk dust with grain sizes of around 10 μm (mean grain size d₅₀).To produce a spray powder, the filler is mixed and compounded withresin, hardener and further additives. The compounded product is thenmilled to form powder.

[0046] These milling processes are usually carried out in appliancesmade from steel or hard metal (Mohs hardness 5-6). The use of hardfillers, e.g. silica flour (hardness 7), leads to metallic abrasion,preferably in the form of chips in the sub-mm range. These areincorporated in the insulation and, on account of their aciculargeometry, lead to locations where the electric field strength is locallyvery greatly increased, where experience has shown that an electricalbreakdown can occur. The abrasion is avoided by using “soft” fillers(Mohs hardness≦4), e.g. chalk dust, and/or by using relatively finefillers with d₅₀<<1 μm, e.g. clay, SiO₂, ZnO or TiO₂.

[0047] Furthermore, fine fillers of this type have the advantage that,even if defects such as cavities or metallic inclusions are present,they prevent or at least very considerably delay electricalbreakthroughs, as disclosed, for example, in U.S. Pat. No. 4,760,296, inthe name of Johnston et al., or in German patent application DE 4037 972A1. In these two publications, the effect of increasing the service lifeis achieved by completely or partially replacing the coarse filler withfillers which have grain sizes in the nanometer range (0.005 to 0.1 μmmaximum grain size). However, nano fillers have the unacceptableadditional property of greatly increasing the melt viscosity of thepowder mixture, known as the thixotropy effect. This causes problemsboth during production of the powder and during processing thereof.Nevertheless, nano fillers represent a usable alternative for increasingthe service life. However, for the application according to theinvention it has also been found that the alternative of using TiO₂powder with mean grain sizes of approx. 0.2 μm to completely orpartially replace coarse fillers does not lead to a disadvantageousincrease in the melt viscosity yet nevertheless does produce the effectsof increasing service life in the same way as nano fillers. Theproportion of TiO₂ powder in the total mixture should be at least 3%,preferably at least 5%.

[0048] Conductive layers which are used for internal corona-dischargeprotection and external corona-discharge protection can be produced bythe use of conductive fillers, such as for example graphite, carbonblack and/or metal powder.

[0049] The process sequence involved in the novel insulation ofelectrical conductors in accordance with the invention will now bedescribed below on the basis of the fundamental explanations of thematerials and of the device which have been presented above.

[0050] The process comprises the following steps:

[0051] 1) Mounting the coil or bar which is to be coated on a rotationdevice

[0052] In a first step S1, a bar or coil which is to be coated ismounted on a rotation device. In this case, the bar ends or coil eyeletsare the holding points. The bar or coil is advantageouslyprestrengthened by internal adhesive bonding of the conductors or bywinding a tape around them, since this facilitates handling, but this isnot an imperative condition. In the case of relatively large objects,intermediate supports are useful in order to ensure reliable securing onthe rotation device and accurate positioning. The rotation device withthe coil or bar mounted on it is actuated by means of a control device,which is advantageously included in the control unit of a spray device.

[0053] 2) Heating of the bar or coil

[0054] In the second step S2, the bar or coil is connected to electricalheating. This electrical heating may, for example, be resistance heatingproduced by direct current or low frequency, e.g. 50 Hz, or inductiveheating by means of medium frequency or high frequency. This heating isused to heat the bar or coil to a desired substrate temperature.

[0055] 3) Orienting the bar or coil position with respect to the spraygun

[0056] In the following third step S3, the bar or coil position isoriented in order for the spraying to start. During this step, one ofthe flat sides of the bar or coil is oriented perpendicular to the spraygun.

[0057] 4) Spraying an internal corona-discharge protection onto the baror coil

[0058] There then follows, as the fourth step S4 and therefore the firstactual coating step, spraying of an internal corona-discharge protectionin horizontal paths. The movement of the spray gun takes place in such amanner that a homogeneous layer thickness profile is produced. For thispurpose, it may be necessary, in the case of very wide bars, to spray aplurality of overlapping paths in parallel. In this case, a conductiveor semiconducting thermoset is used as the coating powder which issprayed on. To accelerate the spraying, in the case of large objects itis possible to use a plurality of spray guns at the same time. Theintermediate supports used to stabilize the position of relatively largeobjects move away automatically when the spray gun approaches, in orderto allow complete coating of the bar or coil. It is easy to vary thelayer thickness applied to the bar or coil by means of the quantity ofcoating powder which is delivered and the rate of movement of the spraygun. The layer thickness is generally≦0.2 mm per pass, in order toensure that the layer is in each case free from bubbles.

[0059] 5) Rotation of the bar or coil and repetition of steps S3 and S4

[0060] As the following fifth step S5, after the coating of a flat sidehas ended, the bar or coil is rotated, so that a further, as yetuncoated bar or coil side faces the spray gun. Then, the third andfourth steps S3 and S4 are repeated in order to coat the next side ofthe bar or coil. In the same way, the fifth, third and fourth steps S3,S4 and S5 are repeated again for all further sides of the bar or coil,until the bar or coil has been completely coated. Moreover, in the caseof coils the steps are repeated for the other limb of the coil.

[0061] In general, the thickness required for the internalcorona-discharge protection is applied in a single pass; in exceptionalcases, however, it is also possible to carry out a plurality of coatingpasses, if a layer thickness of greater than 0.2 mm is desired.

[0062] 6) Curing of the internal corona-discharge protection

[0063] Then, as step 6, this internal corona-discharge protective layeris cured partially or completely, i.e. for a time of between 2 and 10min or 20 and 60 min at 200° C.

[0064] 7) Application of the insulating layer in accordance with stepsS3 to S6

[0065] Then, in a seventh step S7, the actual insulating layer isapplied. In this case, a different coating powder from the internalcorona-discharge protection is used, specifically an insulant-filled orunfilled thermoset.

[0066] Once again, the third, fourth and fifth steps described above,unlike the steps described above, are carried out only with theinsulating coating powder, with layers of <0.2 mm being appliedrepeatedly until a desired insulation thickness is reached. In thiscase, after each layer has been sprayed on, intermediate curing, whichlasts from 2 to 10 times the gel time of the powder, is carried out. Inaddition, to ensure that the coating powder is melted on reliably, it isnecessary to take account of the fact that the substrate temperature mayhave to be readjusted as the layer thickness increases. This ispreferably carried out without contact, for example by means of an IRpyrometer. To prevent the conductor elements and their insulation frombeing overheated, it is possible to monitor the temperature-dependentresistance of the conductor.

[0067] 8) Application of the external corona-discharge protection inaccordance with steps S3, S4, S5 and S6

[0068] As the next step, which follows the application of the insulatinglayer (step S7), an external corona-discharge protection comprisingconductive epoxy is applied to the insulating layer. The material usedand the process steps correspond to those described in the third, fourthand fifth steps.

[0069] 9) Post-curing of the insulation applied

[0070] The final step after the coating operations (steps S1 to S8) haveended is either post-curing of the insulation which has been appliedusing current heating on the holding device or in the furnace afterremoval from the rotation device.

[0071] Since, in this insulation process according to the invention, thebars are held at their ends, these ends are not coated. However, thisdoes not have any disadvantages, since the bar ends in any case remainclear in order for circular connectors to be soldered on.

[0072] In the case of coils, on the other hand, the region of the coileyelet is intentionally not coated. This is because this ensures thedeformability of the coils which is required for installation. The coileyelets are in this case only insulated in the installed state afterinstallation.

[0073] The following processes are recommended for this operation ofinsulating the coil eyelets:

[0074] 1) Either the motor stator is positioned vertically and the coilsare heated electrically. In this case, the coil eyelets of the lowercoil end are coated with thermoset powder as a result of the coil endsbeing immersed in a fluidized-bed sintering tank. In this case, thestator is rotated through 180° and the coil eyelets of the other statorend are coated in the same way as that described above. This process issuitable in particular for coating relatively small stators.

[0075] 2) Alternatively, the motor winding is heated resistively and theend parts of the coils are insulated by spray-sintering. This process isadvantageously used for relatively large motors, in which the totalweight of the stator is high and vertical mounting may cause problems.Also, in the case of large machines the distances between the coils aregenerally greater, and consequently coating with a spray gun becomeseasier.

[0076] In an alternative embodiment of the process according to theinvention, it is possible to dispense with the step of spraying on aninternal corona-discharge protection as described in the above steps 3)to 5) if a tape, which is provided with a conductive or semiconductinglayer and therefore forms an internal corona-discharge protection at thetime of the prestrengthening, is used for the prestrengthening of thecoil or bar.

[0077] To summarize, the invention discloses a process for producing ahigh-quality insulation for conductors or conductor bundles ofelectrical machines by means of spray-sintering. Unlike in the priorart, it is possible to apply internal corona-discharge protection,insulation and external corona-discharge protection to conductors orconductor bundles in which, on account of the absence of defects, nopartial discharges occur, without the need for complex special equipmentwhich is required for the insulation with mica which has previously beenemployed.

[0078] Therefore, the invention provides a simple, inexpensive processfor insulating conductor bars or coils with a high-quality insulationwhich is free of defects and therefore free of partial discharges.

[0079] To summarize, the present invention discloses a process forproducing a high-quality insulation for conductors or conductor bundlesof electrical machines by means of spray-sintering. Unlike in the priorart, it is possible to apply internal corona-discharge protection,insulation and external corona-discharge protection to conductors orconductor bundles in which, on account of the very low level of defectsin the insulation and the inherent resistance thereof to partialdischarges, it is possible to dispense with the use of winding processesusing glass/mica tapes. This eliminates the use of complex specialequipment and the throughput times can be shortened considerably.

1. A process for producing a high-quality insulation for conductors or conductor bundles, comprising the steps of: (S1) mounting a conductor or conductor bundle which is to be coated on a rotation and holding device, (S2 to S8) applying an insulation with a desired layer thickness profile to the preheated conductor or conductor bundle by means of a spray gun arranged on an adjustable displacement unit, and during this application either a combined movement of spray gun and conductor or conductor bundle is effected by means of the displacement unit and the rotation and holding device, or a movement of the spray gun alone is effected by means of the displacement unit, and (S9) post-curing the layers which have been applied.
 2. The process as claimed in claim 1, comprising the further steps of: (S2) heating the conductor or conductor bundle to a predetermined substrate temperature, (S3) orienting the conductors or conductor bundle position with respect to the spray gun in such a manner that one of the flat sides faces the spray gun, (S4) spraying an internal corona-discharge protection onto the conductor bundle or conductor which is to be coated, (S5) rotating the conductor or conductor bundle in such a manner that a further flat side faces the spray gun, and repeating steps S3 and S4 until all the flat sides of the conductor or conductor bundle have been coated, (S6) partially or completely curing the internal corona-discharge protection which has been applied, (S7) applying an insulating layer in accordance with steps S3 to S6 to all sides of the conductor or conductor bundle, with layer thicknesses of less than 0.2 mm, and carrying out intermediate curing after each layer, the substrate temperature being adjusted if necessary, and (S8) applying an external corona-discharge protection in accordance with steps S3 to S6.
 3. The process as claimed in claim 1 or 2, in which the conductors or conductor bundles are a transposed bar or a coil.
 4. The process as claimed in claim 1, 2 or 3, comprising the further step of (S0) prestrengthening the conductor or conductor bundle by internal adhesive bonding or by winding a tape around it.
 5. The process as claimed in claim 3, in which step S1 comprises holding the bar at the bar ends or the coil at the coil eyelets.
 6. The process as claimed in one of the preceding claims 1 to 5, in which in step S1 relatively large objects are held by additional intermediate supports, in order to ensure accurate positioning.
 7. The process as claimed in one of the preceding claims 1 to 6, in which the conductor or the conductor bundle is electrically heated by resistance heating using direct current or low frequency or by inductive heating using medium frequency or high frequency.
 8. The process as claimed in one of claims 1 to 6, in which instead of the heating in accordance with (S2) heating of the conductor or conductor bundle takes place before or after mounting on the rotation and holding device, by means of irradiation, and subsequent heating takes place whenever the substrate temperature drops below a predetermined substrate temperature.
 9. The process as claimed in one of the preceding claims 1 to 8, in which steps S4, S7 and S8, in the case of wide conductors or conductor bundles, are carried out by spraying in a plurality of parallel paths by means of a plurality of spray guns.
 10. The process as claimed in one of the preceding claims 1 to 9, in which in step S4 conductively or semiconductively filled, thermally crosslinking plastic is used as coating powder.
 11. The process as claimed in claim 6, in which in step S4 the intermediate supports automatically move away when the spray gun approaches.
 12. The process as claimed in one of the preceding claims, comprising the further step of varying the layer thickness by controlling the delivery quantity of the coating powder and the rate of movement of the spray gun.
 13. The process as claimed in claim 3, in which, when a coil is present, step S5 is repeated for the other side of the coil.
 14. The process as claimed in one of the preceding claims 1 to 13, in which in step S7 an insulant-filled, thermally crosslinking plastic is used as coating powder.
 15. The process as claimed in one of the preceding claims 1 to 14, in which in step S8 a conductive, thermally crosslinking plastic is used as coating powder.
 16. The process as claimed in one of the preceding claims 1 to 15, in which in step S9 the post-curing takes place by means of current heating, subsequently the conductor is allowed to cool and is then removed from the rotation device, or the conductor is immediately removed from the rotation device and is post-cured in a furnace.
 17. The process as claimed in claim 4, in which the steps S3 to S6 for application of the internal corona-discharge protection are dispensed with without replacement if, in step S0, a tape which is provided with a conductive or semiconducting layer and forms internal corona-discharge protection at the same time as the strengthening is used.
 18. The process as claimed in one of the preceding claims 1 to 18, in which step S4 comprises charging the coating material before it is applied to the conductor surface, in which case this charging may be electrostatic or triboelectric, so that thicker coating takes place at edges.
 19. The process as claimed in one of the preceding claims 1 to 18, in which in steps S3 to S8 different layer thicknesses can be applied to the different conductor or conductor bundle sides.
 20. The process as claimed in claim 3, comprising the further step of (S10) in the case of a coil, after installation of the coil, positioning the motor stator vertically, electrically heating the coil and immersing the coil eyelets in a fluidized-bed sintering tank for coating with epoxy, in each case for both coil sides, or in the case of a bar, resistively heating the motor winding and insulating the end parts by spray-sintering for both bar ends.
 21. The process as claimed in one of the preceding claims 1 to 20, in which in step S7 the surface temperature is monitored without contact and the substrate temperature is readjusted according to a deviation in the recorded surface temperature from a desired surface temperature.
 22. The process as claimed in one of claims 10, 14 or 15, in which the thermally crosslinking plastic is an epoxy resin in the B state. 